1. Field of the Invention
The present invention relates generally to marine anchors, and more particularly to drag embedment anchors.
2. Background of the Related Art
Drag embedment anchors are generally comprised of two major components, a fluke and a shank. Generally, the fluke is relatively flat and has a large surface area, with two pointed front tips which penetrate the soil on the sea floor as the anchor is dragged. When the anchor is completely embedded in the soil on the sea floor, the pressure of the soil on the fluke is a major component of the holding power of the anchor. A typical fluke is formed from a flat plate stiffened by external ribs, or from a wedged-shaped box stiffened by internal ribs.
The shank is generally a long, thin member which is fastened near the stern of the fluke at one end, and to a mooring line at the other end. In most anchors, the shank is coincident with the central longitudinal axis of the fluke when the anchor is viewed from above. The shank serves to transmit forces between the fluke and the mooring line.
There are two broad categories of drag embedment anchors within which most anchors can be classified. The first category includes traditional or swing shank anchors. Traditional anchors have shanks which are straight and rotatably secured to the fluke at a single hinge point so that the shank can pivot to a limited degree on either side of the fluke. As the anchor is dragged along the sea floor, one side of the fluke will face downward, toward the sea floor. Once the fluke tips penetrate the soil, the shank will swing to the other side of the fluke. Since either side of the fluke can be facing downward, the fluke must be symmetrical in shape.
The second broad category of anchors includes the modern, or fixed shank type. The flukes of these anchors have a defined top surface and underside since the attitude of the shank is fixed relative to the fluke during operation. The shank extends upwardly from the top surface of the fluke. In order to penetrate the soil, these anchors must either land on the sea floor with the fluke beneath the shank and with the fluke's bottom side resting on the sea floor or be designed to achieve this position upon dragging. In order to achieve a high holding power, the anchor must deeply penetrate the soil to a depth where the anchor is securely held. Conversely, the anchor should not penetrate so deeply that the anchor cannot be retrieved.
Anchor designs of both types may be adjusted to accommodate varying soil conditions. The altitude of the shank relative to the fluke should be changed to assist the initial penetration and ultimate depth of the flukes within the soil. The softer the soil is, the greater the attitude should be. However, the means for adjusting the fluke opening angle on previous anchors have suffered from various drawbacks. In swing shank anchors, this angle can be varied by fastening a stopper to the fluke which limits the rotation of the shank past a certain point. However, the stopper is removed when a wide fluke opening angle is desired, and must be stored and handled when not in use. Further, the weight of the stopper reduces efficiency of the anchor when the stopper is in use. Fixed shank anchors may not have an adjustment option or the adjustment may require laborious procedures for securing the shank to the fluke.
An important parameter for measuring anchor performance is the holding efficiency, or the ratio of the holding power to the weight of the anchor. Fixed shank anchors typically have greater holding efficiencies than swing shank anchors which are unnecessarily heavy due the symmetry of the fluke. However, as the holding efficiency of fixed shank anchors increases, the strength of the anchors must be maintained or increased accordingly. In U.S. Pat. No. 5,353,732, Gramet et al. disclose a shank having two parallel legs that are strengthened by stiffening ribs and a stiffening plate. In U.S. Pat. No. 4,397,256, Bruce also discloses a shank having two parallel legs, but with multiple stiffening plates disposed at a positive attack angle to contribute to burial of the anchor. Furthermore, a V-shaped twin shank is disclosed by van den Haak in U.S. Pat. No. 4,706,595 having pairs of crossing elements between the shank legs to increase rigidity.
The performance of the fluke is also a very important determinant of holding efficiency and drag distance. The two fluke tips of a drag embedment anchor are typically coextensive and symmetrically disposed on either side of the fluke axis in order to promote penetration of the anchor. The fluke may have a substantially flat profile or a hollow, wedge-shaped profile, as disclosed by Gramet et al. in U.S. Pat. No. 5,353,732. In U.S. Pat. No. 3,964,421, van den Haak discloses a hollow fluke having longitudinal supporting ribs between the plates forming the top and bottom surfaces of the fluke to increase the strength of the fluke.
However, despite the forgoing developments, there is still a need for a drag embedment anchor that provides an even higher holding efficiency. More particularly, there is a need for a drag embedment anchor that allows faster, smoother and more controlled penetration of the anchor in various soil types. It would be desirable if the anchor could achieve penetration over much shorter drag distances. It would also be desirable if the anchor provided an adjustable angle between the shank and the fluke to allow a single anchor to be used in various soil types.